1. Field of the Invention
The present invention relates to real-time monitoring of chemical and physical interactions between gases and solid surfaces for purposes including detection of molecules, such as airborne molecular contaminants pertaining to manufacturing and processing environments.
2. Statement of the Problem
Many manufacturing processes and technologies are susceptible to molecular contaminants (MC) in the form of airborne or gas-phase molecular contaminants (AMC) and in the form of the related surface molecular contamination (SMC) resulting from chemical interactions between AMC and critical surfaces exposed to the same. Such critical surfaces, called “subject surfaces” herein, are, for example: integrated circuit surfaces, such as resist, silicon, and other semiconductors; wiring surfaces made of tungsten, aluminum, or other metals; silicon dioxide surfaces; optical surfaces; mechanical surfaces; surfaces of hard disks; surfaces of flat panel displays; etc. Detrimental effects of SMC include, for example, changes in the chemical, electrical, and optical qualities of critical surfaces. These detrimental effects decrease product performance and reliability and raise product costs. Some examples of such detrimental effects to the above-mentioned critical surfaces include T-topping of resist—an anomaly that undercuts line geometries and leads to device failures and yield reductions; defective epitaxial growth; unintentional doping; uneven oxide growth; changes in wafer surface properties; corrosion; and decreased metal pad adhesion. Many of these effects become particularly detrimental as line widths smaller than 0.13 microns become commonplace. Further, as wafer sizes increase and as device geometry decreases, the demand for more sensitive monitoring techniques will increase. In the optics industry, SMC is a well-known cause of hazing of optical surfaces. SMC also causes friction problems in certain mechanical devices, such as hard disk drives, since SMC-contaminated surfaces may have a significantly higher coefficient of friction than uncontaminated surfaces. SMC also affects the manufacture of hard disk drives and flat panel displays, which, for reasons known in the art, are typically carried out in a plurality of “mini” clean rooms.
The various AMCs causing detrimental SMC may be grouped into four general categories: acids, bases, condensables, and dopants, otherwise referred to as SEMI F21-95 Classes A, B, C, and D. Some AMCs, though, are of no particular class.
Sources for AMC/SMC include inadequate filtration of recirculated air; cross-process chemical contamination within a bay or across a facility, and recirculated air with inadequate ventilation; outgassing of clean-room materials, such as filters, gel sealants, and construction materials, especially new fabrics; as well as contaminants carried in and exuded by human beings, including their bodies, clothes, and their personal care products. When the fluid is outdoor “make-up” air, the sources of AMC/SMC include automobile exhaust, evapotranspiration from plants, and various industrial emissions. AMC also includes chemical compounds and vapors resulting from chemical breakdown of, and interaction between, the molecules within the AMC from the primary sources. Still other sources include various contaminants emanating from industrial equipment, such as pumps, motors, robots, and containers. Yet other sources include accidents, including chemical spills, and upsets in temperature and humidity controls.
AMC can cause yield losses even when present at concentrations as low as subparts per billion by volume (“ppbv”). Such processes therefore require an ultra-clean, well-monitored environment. Since different types of MC cause harm which may differ in kind and degree, it is helpful to identify the components of MC present in a manufacturing environment.
One existing manufacturing environment monitoring approach involves using one or more piezoelectric sensor (PZS). Piezoelectric sensors provide a signal output the frequency of which varies in response to an applied force. An accumulation of molecular contamination on the surface of a PZS effects a change in the PZS output, thereby making the PZS output indicative of a magnitude of accumulated contamination mass.
An improved PZS-based monitoring system involves using one PZS to measure contamination accumulation (the SMC sensor) and a hermetically sealed PZS as a reference sensor. The two sensors are subjected to the same temperature, but only the SMC sensor is exposed to an accumulation of SMC mass. By interpreting the difference between the two sensor outputs as a measure of molecular contamination, temperature-induced influence over PZS output is negated, thereby providing a sensor output which reflects the accumulated contamination mass without a temperature bias.
Although existing monitors have addressed measurement error due to temperature variation, they continue to suffer from error due to humidity and pressure fluctuation in the monitoring environment. Humidity and/or pressure fluctuations affect the SMC sensor but not the reference sensor because of the hermetic seal. Accordingly, a change in humidity and/or pressure will change the difference signal of the monitoring system independently of any change in the contaminant mass in contact with the SMC sensor, thereby providing misleading data regarding molecular contamination levels. Moreover, existing monitoring systems do not distinguish between the classes and types of contamination molecules, that is the constituents of the molecular accumulation. Instead, subject to the error sources discussed above, existing monitoring systems provide output values responsive to a total accumulation of mass rather than to the accumulation of individual contaminants or classes of contaminants.